y = 2/x (The is the integrand). The derivative is -2/x^2
When calculating the length of a line, you have to integrate sqrt[1 + (dy/dx)^2], which in this case is
sqrt[1 + 4/x^4]. That function is the integrand.

Your last step is wrong.
The line length from a to b is the difference in the values of the INTEGRAL of sqrt[1 + 4/x^4] dx at the two end points.

I don't have a clue abut how to integrate that function, but they are only asking you to set up the integral anyway.